The field of superconductivity has been expanding rapidly in recent years due in large part to the discovery of materials which retain the superconductive property at usefully high current levels, temperatures, and external magnetic fields. Among the more useful materials are Nb.sub.3 Sn and V.sub.3 Ga, both intermetallic compounds having the so-called A-15 crystal structure. While these materials can be made in useful shapes and quantities, they are nevertheless expensive to produce, due to the fact that they are metastable phases which cannot be made by simple chemical processes, and because they are extremely brittle and therefore cannot, once made, be mechanically deformed to any great extent.
The method now most widely used to make Nb.sub.3 Sn and V.sub.3 Ga is the "bronze process", whereby Nb or V is brought into contact with a bronze made up of copper and Sn or Ga, respectively. The composite thus formed is mechanically worked to its desired shape and subjected to a long-term, high temperature heat-treatment, whereby the Sn or Ga in the bronze diffuses through the Cu of the bronze to form Nb.sub.3 Sn or V.sub.3 Ga at the interface(s) between the Nb or V and the bronze.
It being well known in the art that superconductors perform better when the superconductive material is divided between a number of extremely fine wires embedded in a metallic matrix, the aim of the art has been to develop methods of making such multifilamentary conductors. The bronze process described above has been proven capable of modification to suit this goal; an example of a workable process is discussed in U.S. Pat. No. 3,918,998, assigned to the assignee of the present invention. A typical process for the manufacture of a multifilamentary Nb.sub.3 Sn conductor begins with the drilling of a plurality of holes in a Cu/Sn bronze billet for the insertion of Nb rods. This billet is then extruded to a rod, which is then drawn down to fine wire. In some cases it is desirable that even more filaments be produced; this can be done by cutting the rod into a large number of equal lengths at some intermediate size, inserting these into an extrusion can, extruding this assembly and drawing the result to fine wire. The rod may be drawn through a hex-shaped die prior to cutting; if the rod is thus hexed, the lengths pack together in the extrusion can with less wasted space.
In some cases it is desirable that there be provided a quantity of pure copper of good electrical conductivity. This may be done by lining a copper extrusion can with a layer of a metal which is impermeable to tin, during high temperature heat-treatment, so that the tin does not diffuse into the copper and lower its conductivity; tantalum is the metal most commonly used. See, e.g., U.S. Pat. No. 3,996,661. A quantity of a good electrical conductor in close proximity to the superconductive material is useful as an alternate current path or shunt in situations where it is likely that some fraction of the superconductive filaments will return to the normally-conducting state, which can happen, for example, in a rapidly-varying magnetic field.
The present state of the art, as outlined above, uses the bronze process to achieve multifilamentary intermetallic superconductors which are "stabilized" by the provision of a quantity of a good electrical conductor. However, the bronze process is not without its difficulties. Chief among these is the fact that in order to improve the maximum current density carried by the superconductor, it is desirable to increase the amount of superconductive material per unit of cross-sectional area of the whole conductor. To do this it is clear that a sufficiency of tin must be provided, which could be done simply by increasing the percentage of tin in the bronze. Unfortunately, the production of a large number of extremely fine filaments demands a large number of metal-working steps--chiefly drawing--during which the bronze work hardens very quickly, necessitating frequent time-consuming and costly annealing operations. In fact, the practical maximum volume percentage of tin in the bronze which permits working is 15%; and even at this relatively low value, annealing is required rougly every two to six drawing operations, at a rate of 15-20% area reduction per pass.
A solution to this problem is suggested in U.S. Pat. No. 3,838,503. The approach is to simply draw Nb or V wires in a pure copper matrix to the final size desired, and only then adding Sn or Ga to the external surface of the wire, typically by electroplating. Upon heat treatment, the Sn or Ga is diffused through the copper and forms the desired intermetallic compound on the surface of the Nb or V filaments.
This method is not without utility, but is severely limited in that only a very thin layer of Sn or Ga can be applied by conventional dipping, electroplating or vapor deposition processes, thus limiting the size of the conductor which can be produced. An improvement on this method, which has been suggested in U.S. Pat. No. 3,829,963, is to perform a number of such dipping or plating steps and following these by homogenizing steps, thus gradually building up the amount of Sn or Ga in the bronze. However, this process is rather complicated, and is limited as to the size of the conductor which can be effectively permeated with Sn or Ga.
Another alternative is described in U.S. Pat. No. 3,954,572. If it is desired to manufacture Nb.sub.3 Sn, for example, a number of Nb rods will be inserted into a Cu matrix. This assembly is then worked to a fine wire, and a Cu/Sn bronze is electroplated on the surface of the wire. Upon heat-treatment, the Sn diffuses towards the Nb to form Nb.sub.3 Sn. This method is, however, limited by the amount of tin which can be readily applied. A similar method is discussed in Erwens, Fabrication and Properties of Multifilament Nb.sub.3 Sn Conductors, Z. Metallk, 66 (12):711-14 (December 1975); it too is limited in that the maximum thickness of tin which can be applied is approximately 30 .mu.m.
Still another approach to the problem in the past has been to fabricate a precursor with a plurality of Nb tubes embedded in a Cu matrix. The inside of each tube is filled with CuSn bronze or pure Sn or both. After working the precursor to its final size a reaction heat treatment converts the interior wall of each Nb tube to Nb.sub.3 Sn. The problem in this method is the difficulty and/or expense of obtaining the Nb tubes and of maintaining their integrity during the extrusion and wire drawing steps.
Given the state of the art as outlined above, it will be apparent that there exists a distinct need for a method of making a multifilamentary superconductor of the Nb.sub.3 Sn type from readily available materials that does not involve the mechanical working of bronze and that does not require the diffusion of externally applied Sn through the conductor to the embedded filaments.